1. Field of the Invention
The present invention relates to the field of instrument engineering and is specifically concerned with a method and apparatus for calibration of instruments used to count and to determine the size of particles suspended in dispersion media.
The invention may find a successful application in essentially all the fields where determining the size distribution of particles suspended in transparent dispersion media is needed. It may be particularly useful in the chemical, petroleum processing, and pharmaceutical industries for monitoring environmental pollution, contamination of production-process media, and determining the concentration of a suspended product.
2. Description of the Prior Art
The manufacture of novel products, introduction of novel production processes, and implementation of environmental protection programs which involve monitoring the contamination of sewage water and other production-process media by various production wastes as well as analyzing the particle size of plankton in open basins greatly extended the range of disperse systems where the particle size distribution is to be checked promptly.
Thus, for example, in the production of suspended plastics, such as polyvinyl chloride, polymethyl methacrylate, etc., the used disperse fluid is drained at the end of the production process; some amount of the product may remain in the liquid, which may result in contamination of water basins. This necessitates the provision in the production line of an instrument to monitor the concentration of the initial product, contaminating the flow, in order to determine whether the liquid is to be refiltered. Such urgent problems as monitoring the performance of filters for specific media, determining the fineness of grinding of abrasive and other powders, analyzing the particle size of plankton, determining the degree of contamination of various production-process flows by particles of the product being made, as well as monitoring the contamination of hydraulic fluids by particles resulting from wear of rubbing parts are at present solved rather approximately, with an inadequate accuracy, and in many instances still remain to be solved.
Among analytical methods for determining the particle size distribution, an extensive application is at present found by photometric-counting analysis techniques, based on determining the particle size by measuring the amplitudes of electric pulses produced at the photodetector output whenever the particles pass through an illuminated sensing zone. Inasmuch as the measurement is accomplished indirectly, its results depend on the optical properties of the medium under analysis, and therefore the instruments of this type must be calibrated against standard media whose optical properties are identical to those of the medium to be analyzed. The range of calibration monodisperse media known at present is quite limited: latexes; dental powders; glass beads; melamine-formaldehyde resin suspensions. Preparing monodisperse calibration media of a predetermined origin involves complex and labour-consuming processes and in some cases is impossible at all. This considerably restricts the range of media analyzed by photometric-counting instruments.
The restrictions hold also for other counting methods of particle size analysis, such as gravimetric, conductometric, dielcometric, etc.
When no standard media are available, the results of a particle size analysis are arbitrary. If, for example, an instrument has been calibrated with standard particles of glass, then analyses of disperse media where particles of another origin, such as rubber, are present will yield wrong results. To obtain reliable results in such events, labour- and time-consuming microscopic analysis techniques must be resorted to in practice.
A calibration method has been proposed, based on passing calibration media through the illuminated sensing zone of an instrument (Standards for the calibration of automatic particle counters. "Hydraulics and Pneumatics", July, 1955).
Calibration media are prepared from initial standard ones. To this end, a weighed quantity with the same distribution of dispersed phase particles as in the dispersed phase of the standard media is taken from the total mass of particles (or suspension). The part thus taken is diluted in a dispersion medium of a certain volume to obtain the predetermined concentration. The error in preparing a calibration medium of the predetermined concentration depends on the degree of "monodispersity" of dispersed phase particles. The higher the monodispersity, the less particles whose size differs from the average one are in the medium and hence the more accurately the quantity N.sub.o of dispersed phase particles can be determined from the weight or volume of the standard (primary) medium.
The calibration medium thus prepared, which contains a known quantity (determined to within a few percent) of particles of a certain size (in fact, of a range of sizes), is then passed through the sensing zone of the instrument under calibration.
The apparatus should count the number of crossings of the sensing zone by the particles; as the total volume of the calibration medium has been passed through the sensing zone, a number N.sub.1 of pulses which coincides with or is close to the calculated one should be recorded at the recorder output. The sensitivity of the instrument is set so that all the pulses be counted at the threshold being calibrated. It this is not the case, the sensitivity is readjusted until the condition is satisfied.
The highest calibration accuracy is attained with the use of strictly monodisperse standard media.
Unfortunately, no strictly monodisperse media are at present available for the range of media being analyzed, and this makes it difficult to set the threshold of sensitivity of an instrument being calibrated.
There is known a limited range of monodisperse media with particles of various origin, which restricts the field of application of instruments of the above-mentioned purpose.
When stored, the known monodisperse media may coagulate, give rise to aggregates, which changes the value of the certified parameter, and therefore must be periodically re-certified for their re-use as standard ones. Periodical calibration of the instruments' thresholds is needed due to a possible change in the rating of a parameter of one of instrument's units as well as in carrying out preventive and scheduled maintenance jobs.
A grave disadvantage of the method lies in that the calibration process is laborious and takes a long time; this impedes periodical checkout of the stability of thresholds as well as automation of calibration in the field.
Another method for calibration of instruments used in particle counting and sizing applications has been reported in "Journal of the Air Pollution Control Association", October 1968, vol. 18, No. 10, p. 658.
In this method, calibration of an instrument is based on modulating a flux of radiation taken from the port of the illuminator of the instrument under calibration, the modulated flux being directed onto the photodetector of the apparatus; the modulation is accomplished through interrupting the light flux by a rotating disk with calibrated apertures. Rotation of the disk produces a regular sequence of pulses with a certain amplitude, which are used as a basis to readjust the threshold of sensitivity of the instrument being calibrated. Bringing the threshold sensitivity into accordance with a certain particle size is accomplished by a primary calibration with the use of standard monodisperse media.
The primary calibration (of the modulator) proceeds as follows. Calibration media are prepared from standard ones and passed through instruments. An instrument counts the number of crossings of the sensing zone by particles, and a number of pulses whose amplitude is proportional to the size of corresponding particles in the calibration medium will be recorded at the output.
The sensitivity of the instrument is set such that particles of a certain size be recorded at a certain value of the sensitivity threshold. At the same value of sensitivity, the pulses from the modulator have a certain amplitude which is memorized in an electronic unit of the instrument.
The primary calibration of the modulator against standard media is carried out whenever a change in parameters or characteristics of the instrument's units is possible, such as in case of a failure and replacement of the radiation source or receiver, realignment, a change in parameters of the sensitive volume fouling of optical elements, etc.
The pulses from the modulator should in the course of operation have the same amplitude as in the primary calibration. Thus, the threshold sensitivity of an instrument is in the course of operation checked and readjusted against the pulses from the modulator.
However, this method for calibration of particle counting and sizing instruments suffers from a number of disadvantages.
The modulator promotes detecting and compensating for general changes in the instrument's parameters (e.g. a change in the radiation flux, fouling of optical elements, a change in the recorder's sensitivity), but is incapable of revealing possible local variations or redistributions of the illuminance in the sensing zone, which may arise in the course of operation, such as contaminations of the tray walls, existence of dark zones, due to a realignment, etc.
To analyze media of a certain origin, the modulator (the instrument's sensitivity) must be calibrated against monodisperse calibration media of the same origin. For another parameters or origin of the medium under analysis, the instrument readings will be not true. It follows that to obtain reliable particle size analysis results, the instrument must be calibrated against standard calibration media where the dispersion medium and the dispersed phase have the same properties as does the medium under analysis; but this is attainable very infrequently because of a limited range of standard systems, which narrows the field of application of such instruments.
When monodisperse standard media are available, the modulator is calibrated by comparing in amplitude the pulses from particles in the calibration media with the pulses of the modulator. The higher the degree of monodispersity of particles, the simpler and more accurate is the calibration process. No strictly monodisperse calibration media exist, however, and therefore the calibration is based on the average size of dispersed phase particles, obtained by statistically averaging the sizes of a great quantity of particles; this gives rise to a certain error in the primary calibration of the threshold of sensitivity of an instrument and of an modulator.